Evaluating the effectiveness of a longitudinal magnetic field in reducing underdosing of the regions around upper respiratory cavities irradiated with photon beams--a Monte Carlo study

Impact of the cavity on sinus wall dose in magnetic resonance image-guided radiation therapy

PURPOSE

This study aims to investigate the impact of the cavity on the sinus wall dose by comparing dose distributions with and without the sinus under magnetic fields using Monte Carlo calculations.

METHODS

A water phantom containing a sinus cavity (Empty) was created, and dose distributions were calculated for 1, 2, and 4 irradiation fields with 6 MV photons. The sinus in the phantom was then filled with water (Full), and the dose distributions were calculated again. The sinus was set to cubes of 2 cm and 4 cm. The magnetic field was applied to the transverse and inline direction under the magnetic flux densities of 0 T, 0.35 T, 0.5 T, 1.0 T, and 1.5 T. The dose distributions were analyzed by the dose difference, dose volume histogram, and D₂ with sinus wall thicknesses of 1 and 5 mm.

RESULTS

D₂ in the "Empty" sinus wall under transverse magnetic fields for the 1-field and 4-field cases was 51.9% higher and 3.7% lower than that in the "Full" sinus wall at 1.5 T, respectively. Meanwhile, D₂ in the Empty sinus wall under inline magnetic fields for 1-field and 4-fields was 2.3% and 2.6% lower than that in the "Full" sinus at B = 0 T, respectively, whereas D₂ was 0.9% and 0.7% larger at 1.0 T, respectively.

CONCLUSIONS

The impact of the cavity on the sinus wall dose depends on the magnetic flux density, direction of the magnetic field and irradiation beam, and number of irradiation fields.

Influence of internal fixation systems on radiation therapy for spinal tumor

In this study, the influence of internal fixation systems on radiation therapy for spinal tumor was investigated in order to derive a theoretical basis for adjustment of radiation dose for patients with spinal tumor and internal fixation. Based on a common method of internal fixation after resection of spinal tumor, different models of spinal internal fixation were constructed using the lumbar vertebra of fresh domestic pigs and titanium alloy as the internal fixation system. Variations in radiation dose in the vertebral body and partial spinal cord in different types of internal fixation were studied under the same radiation condition (6 MV and 600 mGy) in different fixation models and compared with those irradiated based on the treatment planning system (TPS). Our results showed that spinal internal fixation materials have great impact on the radiation dose absorbed by spinal tumors. Under the same radiation condition, the influence of anterior internal fixation material or combined anterior and posterior approach on radiation dose at the anterior border of the vertebral body was the greatest. Regardless of the kinds of internal fixation method employed, radiation dose at the anterior border of the vertebral body was significantly different from that at other positions. Notably, the influence of posterior internal fixation material on the anterior wall of the vertebral canal was the greatest. X‐ray attenuation and scattering should be taken into consideration for most patients with bone metastasis that receive fixation of metal implants. Further evaluation should then be conducted with modified TPS in order to minimize the potentially harmful effects of inappropriate radiation dose.

PACS number: 87.55.D‐

Dosimetry of interface region near closed air cavities for Co-60, 6 MV and 15 MV photon beams using Monte Carlo simulations

Underdosing of treatment targets can occur in radiation therapy due to electronic disequilibrium around air-tissue interfaces when tumors are situated near natural air cavities. These effects have been shown to increase with the beam energy and decrease with the field size. Intensity modulated radiation therapy (IMRT) and tomotherapy techniques employ combinations of multiple small radiation beamlets of varying intensities to deliver highly conformal radiation therapy. The use of small beamlets in these techniques may therefore result in underdosing of treatment target in the air-tissue interfaces region surrounding an air cavity. This work was undertaken to investigate dose reductions near the air-water interfaces of 1×1×1 and 3×3×3 cm³ air cavities, typically encountered in the treatment of head and neck cancer utilizing radiation therapy techniques such as IMRT and tomotherapy using small fields of Co-60, 6 MV and 15 MV photons. Additional investigations were performed for larger photon field sizes encompassing the entire air-cavity, such as encountered in conventional three dimensional conformal radiation therapy (3DCRT) techniques. The EGSnrc/DOSXYZnrc Monte Carlo code was used to calculate the dose reductions (in water) in air-water interface region for single, parallel opposed and four field irradiations with 2×2 cm² (beamlet), 10×2 cm² (fan beam), 5×5 and 7×7 cm² field sizes. The magnitude of dose reduction in water near air-water interface increases with photon energy; decreases with distance from the interface as well as decreases as the number of beams are increased. No dose reductions were observed for large field sizes encompassing the air cavities. The results demonstrate that Co-60 beams may provide significantly smaller interface dose reductions than 6 MV and 15 MV irradiations for small field irradiations such as used in IMRT and tomotherapy.

Evaluation of underdosage in the external photon beam radiotherapy of glottic carcinoma: Monte Carlo study

PURPOSE

Underdosage in the human larynx may be the true factor behind the decrease in local control rates.

PATIENTS AND METHODS

To evaluate underdosage with Monte Carlo a CT-based geometrical model of the patient's neck (mathematical neck) was created. Dose was calculated for a pair of 6 Me V parallel-opposed photon beams modulated with 15 degree steel wedges.

RESULTS

At least 5% of volume of 3.5 cm(3) hypothetical tumor near the air wall of the larynx receives less than 86% of the maximum tumor dose. The same volume received less than 91% of the maximum tumor dose when the mathematical neck had no air cavities.

CONCLUSIONS

We conclude the significant underdosage at the air-tissue interface in the larynx occurs in traditional radiotherapy treatments, especially in the glottic part of the larynx.

Magnetic confinement of electron and photon radiotherapy dose: A Monte Carlo simulation with a nonuniform longitudinal magnetic field

It recently has been shown experimentally that the focusing provided by a longitudinal nonuniform high magnetic field can significantly improve electron beam dose profiles. This could permit precise targeting of tumors near critical areas and minimize the radiation dose to surrounding healthy tissue. The experimental results together with Monte Carlo simulations suggest that the magnetic confinement of electron radiotherapy beams may provide an alternative to proton or heavy ion radiation therapy in some cases. In the present work, the external magnetic field capability of the Monte Carlo code PENELOPE was utilized by providing a subroutine that modeled the actual field produced by the solenoid magnet used in the experimental studies. The magnetic field in our simulation covered the region from the vacuum exit window to the phantom including surrounding air. In a longitudinal nonuniform magnetic field, it is observed that the electron dose can be focused in both the transverse and longitudinal directions. The measured dose profiles of the electron beam are generally reproduced in the Monte Carlo simulations to within a few percent in the region of interest provided that the geometry and the energy of the incident electron beam are accurately known. Comparisons for the photon beam dose profiles with and without the magnetic field are also made. The experimental results are qualitatively reproduced in the simulation. Our simulation shows that the excessive dose at the beam entrance is due to the magnetic field trapping and focusing scattered secondary electrons that were produced in the air by the incident photon beam. The simulations also show that the electron dose profile can be manipulated by the appropriate control of the beam energy together with the strength and displacement of the longitudinal magnetic field.

Effect of ethmoid sinus cavity on dose distribution at interface and how to correct for it: magnetic field with photon beams

Recent work proposed the use of magnetic field as a solution to reduce the undesirable effect of air cavities on dose after the air/tissue interface. In contrast to the published work that looks into the problem with slab geometries, in this work we use actual anatomy based on CT images and the magnetic flux from a Helmholtz coil-pair configuration to investigate the problem and to evaluate the efficacy of the proposed solution. The EGS4 phantom was created using CT scans of the head at the level of the ethmoid sinus. The sinus measures 1.95 x 2.18 x 2.00 cm3. The grid size used is 0.15 x 0.15 x 0.4 cm3. Three different radiation beams, 1 x 1, 2 x 2, and 4 x 4 cm2, all 6 MV irradiate the phantom in two different configurations: single beam and parallel opposed. The magnetic field has three different strengths: 0.0, 0.5, and 1.0 T. These represent the maximum strength achieved in the middle of the configuration, between the two coils. The depth of the second buildup region in the absence of the magnetic field was used as the normalization point for the purpose of analysis. Dose was then scored at 0.23 cm after the air/tissue interface. A second phantom, very similar to the CT-based phantom, was created, but with the sinus cavity filled with unit-density tissue; everything else remained the same. This phantom provides a base to investigate the effect of the air cavity on dose. The phantom was termed the phantom without air, or PWA for short. We use the terms "dose reduction ratio" (DRR), defined as one minus the ratio of the dose in PWA to the dose with the presence of the cavity multiplied by 100% and the "dose improvement ratio" (DIR), defined as the ratio of dose with B to that without B, to evaluate the reduction in dose due to the cavity and the improvement in dose with magnetic field, respectively. For single beam geometry, the reduced dose ranged from 41% (1 x 1 cm2 beam) to less than 2% (4 x 4 cm2 beam). For the same single beam geometry, DIR ranged from 1.13 to 1.00 (DIR = 1 indicates no change) with 0.5 T, whereas it ranged from 1.44 to 1.05 for 1.0 T magnets. When an opposing beam was used, the reduced dose was not as severe, such that DRR ranged from 24% to less than 2%. Whereas the dose improvement ranged from 1.08 to 1.00 for 0.5 T, and from 1.23 to 1.01 for 1.0 T.

Magnetic fields with photon beams: use of circular current loops

Strong transverse magnetic fields can produce very large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. Through EGS4 Monte Carlo simulations, we have examined the effects of applying a magnetic field produced by a pair of circular current loops to a photon beam penetrating a water phantom of finite thickness. We have indeed found very substantial localized dose enhancements, albeit with no corresponding dose reduction just distal to the region of dose enhancement. (However, dose reduction does occur near the distal end of the phantom.) We have also observed two phenomena to be concerned with, for this configuration: significant broadening of the penumbra close to the current loop, and narrowness of the enhanced dose region in a plane parallel to the planes of the loops. We have also examined the use of a single current loop to produce the magnetic field, and have found great asymmetry in the dose distribution; this asymmetry appears to make it impossible to treat with a single circular magnet a tumor of large dimension extending below the application surface.